This is really the best answer, I think. Except the agreement you have to sign to take Visa cards used to stipulate that you could not offer a discount for cash.

Visa’s rules have changed in favor of the merchants at last…… Visa has updated their Visa Core Rules and Visa Product and Service Rules, and the rules changed benefit all business owners in all 50 states ! …Now, you can offer a discount when your customer pays with cash in efforts to incentivize your customers and avoid having to pay credit card processing fees. (Therefore if you sell a product or service for $100, you get to finally keep all $100 of the sale….Not $96.00 )